[Patent document 1] JP S59-214358 A
[Patent document 2] JP 2001-69181 A
[Patent document 3] JP 2008-257221 A (US 2008/0225173)
[Patent document 4] JP S61-195453 A (U.S. Pat. No. 5,001,642 B1)
[Patent document 5] JP H11-317675 A (U.S. Pat. No. 6,405,338 B1)
First, in cases that a clock is extracted from reception data in a digital data transmission, it becomes difficult when the reception data contains consecutive same values. This may be overcome by using Manchester code. On the other hand, Manchester code needs a band twice the amount of information. There is proposed a technology of converting the binary data to easily execute the clock extraction by combining 4B5B conversion and NRZI coding such as 100 BASE-FX used for Ethernet (registered trademark), for instance.
The NRZI code makes the clock extraction easier since the waveform continuously changes when the data value “1” continues. In contrast, when the data value “0” continues, the waveform does not change. Thus, how to delete the pattern containing the consecutive data values “0s” is an issue. Patent document 1 discloses a method to convert a 4-bit data (0x-Fx) into a 5-bit data that has one data value “0” at a head portion and the maximum two data values “0s” at a tail portion, thereby setting the maximum bit length undergoing no edge to “4.” Herein, the edge is a variation between “0” and “1.” This method does not sufficiently suppress the maximum bit length. In addition, there is no special regularity about commands used for communication; the pattern is assigned to each command individually. The number of the commands which can be used are naturally limited.
Patent document 2 discloses a method to remove a command from a target for coding, thereby converting into a 5-bit data containing the maximum one data value “0” at a head portion and also the maximum one data value ‘0” at a tail portion. The maximum bit length undergoing no edge is thereby set to “3.”
The method of Patent document 2 needs to express a frame other than data or a delimiter (one classification of a command) that indicates a boundary of the frame, only by using a specific pattern. Thus, the method of Patent document 2 limits setting of command codes.
Second, there is known a data communication method which assigns a data frame with an identifier to identify the data frame and transmits the data frame to a communication path. For example, Patent document 3 discloses a technology in which a shorter lengthened identifier is assigned to a shorter lengthened data frame, whereas a longer lengthened identifier is assigned to a longer lengthened data frame.
However, in the technology by Patent document 3, when the communication frequency of the longer lengthened data frame is higher than the communication frequency of the shorter lengthened data frame, the communication frequency of the longer lengthened identifier is higher than the communication frequency of the shorter lengthened identifier. This decreases the communication efficiency of the communication network as a whole.
Third, there is conventionally widely known CAN (Controller Area Network) as LAN (Local Area Network) that requires a high reliability (for example, refer to Patent document 4). CAN secures a reliability in the bit rate below 500 k bps using a stack error detection, a bit error detection, a form error detection, an acknowledge error detection, a CRC (Cyclic Redundancy Check) error detection, etc.
Further, there is recently a trend in the communication system to require a data communication speed improvement (several 100M bps or greater) and a high reliability. It is not so easy for CAN to provide a sufficient reliability as a communication system providing a non-collision or clock reproduction necessary for the data communication speed improvement, because of the specific frame structure in CAN.
Fourth, there is known a data communication in which a transmission side transmits a data frame attached with an error detection code such as a CRC, and a reception side detects an error of the reception data using the error detection code and requests re-transmission of the relevant communication frame if detecting an error (for example, refer to Patent document 5).
Such an error detection is only applied to the data but not applied to a header portion of the data frame since the header portion has fewer bits than the data portion and is not assumed to undergo any error detection.